1. Field of the Invention
The present invention relates to ink jet printing apparatus of the continuous type and more specifically to improved systems (structures and modes) of such apparatus that provide self-cleaning for its jet orifice plate.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream from an orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream (e.g. vibrations by an electromechanical transducer) to cause the stream to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream break-off point to impart electrical charge in accord with a print information signal and charged droplets are deflected from their nominal trajectory. In one common binary printing apparatus charged droplets are deflected into a catcher assembly and non-charged droplets proceed along their nominal trajectory to the print medium.
The components described above (particularly the orifice plate and charge plate) must be precisely sized and positioned to achieve accurate droplet placement on the print medium. However, even after such careful manufacture, significant problems are often presented when the apparatus is shut down for extended periods (e.g. overnight). That is, ink residue which remains from previous usage will often dry in the print head during such shut-down periods. If the dried residue is in the orifice plate it can cause crooked jets. If dried ink residue is in the print head cavity it can become dislodged during printing operation and cause blockage of an orifice or a crooked jet. Dried ink residue in other parts of the circulation system can be filtered; however, excessive quantities of such residue necessitates frequent filter maintenance.
Prior art solutions to the ink residue problems have included (i) purging the ink cavity and orifice plate with air upon shut-down of an operational cycle; and (ii) introduction of cleaning solution at start-up and or shut-down.
These solutions are all helpful but not without related difficulties or disadvantages. For example, purging the ink system with air and/or a cleaning solution adds considerable complexity to the apparatus and requires a lengthy flushing period at start-up.
U.S. application Ser. No. 06,722,551, entitled "Ink Jet Printing Apparatus Having a Wet-Storage System", and filed Apr. 12, 1985, in the name of M. Piatt, discloses a highly useful approach for dealing with the above-noted problems. This approach provides a unique home station into which the station into which the apparatus print head assembly is transported from the operative printing path for wet storage and start-up procedures or for periodic maintenance. The present invention provides further improvements in the approach described in the aforementioned copending application and in particular provides structure and operational modes which effect enhanced cleaning of the ink jet orifices and print head cavity.